To date, geneticists have identified plenty of genetic markers that are correlated with intelligence. Taken individually, however, these account for a paltry 1% of the variation in IQ scores. But now, an international team of scientists has identified a set of genes that appear to amplify each other's effects — resulting in a synergistic boost in intelligence.

The finding could change our sense of how intelligence works in the brain – and how it might be improved. But the study has its critics.

In the past, scientists have identified some 20 genetic markers that seem to be linked to intelligence. But none of those genetic markers have turned out to be a "magic bullet" for brainpower — at most, any one of those 20 markers tends to have very subtle effects.

Writing in Science Now, Moheb Costandi describes how neurologist Paul Thompson of the University of California devised a new large-scale strategy for tackling this problem. To this end, he co-founded the ENIGMA network in 2009 –- an international consortium of researchers who combine brain scanning, genetic data, and standardized IQ tests to study brain structure and function.

Their first breakthrough came earlier this year when they identified genetic variants associated with head size and the volume of the hippocampus (a brain structure crucial for learning and memory). The researchers discovered that one of the variants was responsible for a boost of 1.29 points on IQ tests for those carrying it -– a difference they believed was significant.

Costandi explains how the study was done:

The researchers have now used the same strategy to identify more genetic variants associated with brain structure and IQ. In the new study, they analyzed brain images and whole-genome data from 472 Australians, including 85 pairs of identical twins, 100 pairs of nonidentical twins, and their nontwin siblings. They identified 24 genetic variations within six different genes, all of which were linked to differences in the structural integrity of major brain pathways. "We measured the insulation of the neural pathways," says Thompson. "This affects how fast nervous impulses are routed around the brain. If the pathways are insulated poorly, the brain functions less efficiently and is less resistant to disease."

Interestingly, many of the genes were already known, but hadn't previously been linked to brain integrity.

Moreover, the researchers found that some of the variants were associated with intelligence — and that those carrying these variations were doing several points better on IQ tests. What's interesting is that the variants appear to amplify each other's effects, so that possessing more than one provides a boost in intelligence. According to Thompson, "We found a whole range of genetic variants that affect the impact of other variants and we are beginning to understand the guiding principles of these gene networks."

But as Costandi notes, not everyone is buying into the study.

Geneticist Silvia Paracchini feels that the researchers were looking at oversimplified statistics, and that their sample size was too small.

Further, epidemiologist Sarah Medland complains that there was no replication done in the study, and that other appropriate data sets should have been used. She also contends that it's rare to see both IQ scores and brain imaging data used for such a study, and that the data is probably not comparable.

Regardless, Thompson's approach suggests that there's still much to learn about how genetic factors play and feed off each other. It's safe to suggest that there is no such thing as an "intelligence gene". What's more likely is a confluence of genetic factors, the details of which still need to be understood.